Oxygen Sensors

Oxygen sensors are the PCM's primary feedback mechanism for fuel control. They measure the oxygen content in the exhaust stream and report back to the PCM whether the air-fuel mixture is running rich or lean. Without functioning oxygen sensors, the PCM cannot maintain correct fuel mixture and fuel economy, emissions, and driveability all suffer.

Upstream sensors — also called pre-catalyst or Bank sensors — are located in the exhaust before the catalytic converter. These are the sensors the PCM uses for fuel trim control. They switch rapidly between rich and lean dozens of times per minute as the PCM adjusts fuel delivery. Downstream sensors — also called post-catalyst sensors — are located after the catalytic converter. Their job is to monitor converter efficiency. A properly working converter produces a nearly flat downstream sensor signal. If the downstream sensor starts switching rapidly like the upstream sensor, the converter is no longer doing its job — the converter has failed.

Older vehicles use narrowband oxygen sensors that switch between approximately 0.1 volts lean and 0.9 volts rich. Modern vehicles increasingly use wideband air-fuel ratio sensors that provide a precise linear voltage proportional to the exact air-fuel ratio. Wideband sensors are more accurate and allow tighter fuel control. They also test differently — a wideband sensor does not switch the same way a narrowband does. Know which type you are testing before evaluating the signal.

Lazy sensor — switches between rich and lean too slowly. The PCM fuel corrections lag behind the actual mixture. Stuck rich — sensor reads high voltage constantly. PCM adds too little fuel thinking the mixture is already rich. Stuck lean — sensor reads low voltage constantly. PCM adds too much fuel thinking the mixture is lean. Contaminated — silicone from RTV sealant or coolant from a head gasket leak coats the sensor element and prevents it from reading correctly. A contaminated sensor looks different from a worn one — check for the cause of contamination before just replacing the sensor.